1. Technical Field
This presentation discloses an invention relating to a reader for an electromagnetic radiation detection element and an image sensor including such a reader. The invention also relates to a read process that could be implemented by this reader.
An image reader means a device capable of transforming an electronic signal supplied by one or several electromagnetic radiation detection elements into a signal that can be used to represent or record an image, or at least an element of an image (pixel).
The invention is used for applications in different types of image sensors, for example such as cameras. These image sensors may be equipped with detection elements in the form of field effect diodes or elements (MOSFET). In particular, the invention may be used for devices equipped with a sensitive retina integrated in thin layers (TFA Thin Film on Asic).
The invention is also used for applications in the instrumentation (spectroscopic, etc.) and robotics (sensors) fields.
2. State of the Art
The state of the art closest to the invention is described in document (1), for which the complete reference is given at the end of the description. It is also illustrated in FIG. 1, extracted from document (1) and described below to facilitate understanding of the rest of the presentation.
FIG. 1 shows an electrical diagram of a reader 10 associated with an electromagnetic radiation detection element. The detection element shown in the form of a diode 12 is associated with an impedance matching circuit 14 or an input stage, designed essentially to polarize the diode and collect the charges that are generated in it by radiation. The detection element, in the event diode 12, is capable of converting electromagnetic radiation, for example visible or infrared light, or y radiation, into a current of electrical charges. The charge current denoted Iph in the figure is transmitted through the impedance matching circuit 14 to the reader 10 itself. (The reader 10 is usually also called the preamplifier-integrator).
The reader 10 comprises integration means in the form of a capacitor 16. The capacitor is charged by the electronic current Iph in an integration phase and there is a voltage denoted Vint at its terminals. The voltage Vint represents the intensity of the radiation flux received by the detection element during the integration phase.
An amplifier 18 connected to a terminal 20 of the capacitor 16 supplies a detection signal that varies linearly with the voltage Vint. The amplifier in particular connects the terminal 20 of the capacitor to a read signal bus 22. The bus 22 is considered as being a constituent of an output stage from the reader or as forming part of such an output stage.
The terminal 20 of the capacitor 16 is also connected to the inverter input of a comparator 24. The comparator is designed to compare the voltage Vint with a set voltage Vcomp and to control opening of a switch 27 when the voltage Vint goes above the set voltage threshold Vcomp. When the switch 26 is open, the reader 10 is isolated from the detection element 12.
Opening the switch 27 also interrupts integration of the current in the capacitor 16, and prevents a reader saturation phenomenon when the radiation flux received by the detection element is very intense.
It can be seen that the rate of the comparator 24 is controlled by a xe2x80x9cClockxe2x80x9d signal such that opening the switch 26 also has to be validated by a synchronization pulse.
Synchronization pulses are also produced for initialization of the reader before each new integration phase. These pulses then temporarily close a switch 28 connected between the terminals of the capacitor 16, to discharge it.
According to one interesting feature of the reader, it also comprises a timed indication stage 30 capable of outputting a signal representing the duration of each integration phase. Essentially, this signal is provided by a second capacitor 36 connected to a time signal bus 32 through an amplifier 38. The capacitor 36 is connected to a charge voltage source Vramp through a switch 37, which is also opened under the control of the comparator 24.
By combining the sampled voltage blocked by the time indication stage 30 (integration time Tint) and the voltage integrated in the capacitor 16 (integrated photocharge Qint), it is then possible to obtain the value       I          p      ⁢              xe2x80x83            ⁢      h        =                    Q        ⁢                  xe2x80x83                    int                      T        ⁢                  xe2x80x83                    int      
and consequently the incident flux.
A more complete description of the operation of the device in FIG. 1 is given in document (1) mentioned above, although it is not necessary for a good understanding of the invention.
Image sensors equipped with a device like that described above can significantly increase the speed at which images are taken.
The purpose of the invention is to propose a reader operating essentially on the principle of the reader described above, and which has improved operating characteristics.
One purpose in particular is to propose such a reader with low reading noise.
Another purpose is to propose such a reader with improved sensitivity.
There is another purpose related to the previous purpose, which is to propose such a reader that can operate with a low capacity integration capacitor, and that can therefore reduce the surface area of the integration capacitor.
Another purpose of the invention is to propose a read process and a sensor, or detection device, using one or several readers conform with the invention.
In order to achieve these purposes, more precisely the invention relates to a reader for an electromagnetic radiation detection element comprising:
at least one electrical charge integration capacitor that can be connected to at least one electromagnetic radiation detection element, to output a voltage that it is a function of the electrical charges supplied by the said detection element,
an amplifier connected between the capacitor and an output stage from the reader, the amplifier having a high impedance terminal connected to the integration capacitor and a low impedance terminal connected to the output stage, and
comparator means for isolating the integration capacitor from the detection element when the voltage at the capacitor terminals is greater than a set value.
According to the invention, the comparator means are connected to the low impedance terminal of the amplifier.
Since the comparator means are connected to the low impedance terminal of the amplifier, in other words to the terminal connected to the output stage, their input capacitance is not in parallel with the integration capacitor. The result is an increase in the current-voltage conversion factor, creating better sensitivity.
The available voltage at the output from the amplifier reflects the voltage at the capacitor terminals. Therefore, it is used to determine whether or not it exceeds a given value.
Furthermore, the amplifier does not amplify an input noise inherent to the comparator means and to a comparison voltage source applied to these means, if any. This can significantly reduce noise transferred to the output stage.
According to an improvement to the reader, a switch can be connected between the low impedance terminal of the amplifier and the comparator means, to connect the comparator means to the amplifier during an integration phase and to isolate the comparator means from the amplifier during a read phase.
Due to this improvement, the comparator means may be completely isolated from the amplifier and from the output stage to protect the output stage from noise generated by these means.
According to another improvement of the reader, an additional switch may be connected between the amplifier and the output stage, to connect the output stage to the amplifier in a read phase and to isolate the output stage from the amplifier in an integration phase.
This improvement has a number of advantages.
During the integration phase, the low impedance output from the reader is charged solely by the input capacitance in the low capacitance comparator, which gives a low reader polarization current and hence low consumption of the device. On the other end, during the read phase, the low impedance output of the reader is connected to the output stage through a high capacitance bus. The result is a high polarization current to charge the output stage, but only for a short instant.
The final result is a reduction in the electrical consumption.
A reader according to the invention may be common to several radiation detection elements. However, it is preferable to associate an individual reader with each image sensor detection element. One possibility for high level integration of the reader made available as a result of the invention, is that radiation detection elements and readers can be associated with each other. They are associated in a retina in which each elementary point comprises a detection element and a reader.
In one particular embodiment of the amplifier used to apply an input to the output stage, the output stage may be built around a field effect transistor polarized as a follower source. The transistor is polarized by a low intensity polarization current source and may be connected elsewhere to a read bus that forms the output stage, or at least part of it. In this case, the polarization current source is connected in parallel with a polarization source of the read bus, if any.
The invention also relates to a process for reading a detection element, for example a quantic or thermal detection element, using a reader like that described. The process comprises an alternating succession of integration phases and read phases. The comparator means are connected to the amplifier during integration phases, and the comparator means are isolated during read phases.
According to another aspect, it is also possible to connect the amplifier to the output stage during read phases, and to isolate it from the output stage during integration phases, either as a complementary or alternative feature.
Other characteristics and advantages of the invention will become clear from the following description with reference to the figures in the attached drawings. This description is given for illustrative purposes only and is in no way limitative.